I put an RH42 behind the 318 in a new Dart project (1970). I wanted to keep control of the OD and Lock-up as simple as possible to start with. I ran all three wires from the trans to a box on my console and installed 2 switches from New Wire Marine (Rocker Switch Pros) in the box. I ran the wire that provides 12v to the trans to a switched fuse block in the console, ran the two ground wires from the trans through the two switches and then to a good ground. Without the wires connected the trans works great as a 3 speed - no issues at all. I hooked all the wires up a few days ago and have been driving itaround to see how it works. The two switches are back lit with separate wiring and have a indicator light that is supposed to come on when the switch is energized. For reasons beyond my comprehension, although the Overdrive and Lock-up seem to function properly (at 50 mph drop from 2000 to 1500 in OD & 2000 to 1800 with lock-up - 2.94 gears), they do so when the switch indicator light is off, and are disabled when the indicator light is on!!! Are the solenoids in the trans something that locks out these functions when it is energized, rather than enabling those functions???
